Question

Calculate the energy change for the reaction

Answer 1

The change in energy is the energy after the reaction - the energy before the reaction. We can easily find these two energies because of the table above. The energy before the reaction is the energy of one H2 molecule + the energy of one Cl2 molecule. The energy after the reaction is the energy of TWO HCl molecules because you get 2 molecules after the reaction. The total energy change will then be (862-436-242=) 186kJ

Answer 2

`\text{Enthalpy Change} \; \Delta H= E\text{(Bonds Broken)} - E\text{(Bonds Formed)}`

• Bonds broken:
  `1 * \text{H-H}` and
  `1 * \text{Cl-Cl}`;
• Bonds formed:
  `2* \text{H-Cl}`

`E(\text{Bonds Broken}) = 436 + 242 = 678 \; \text{kJ} \cdot \text{mol}^(-1)`

`E(\text{Bonds Formed}) = 2 * 431 = 862 \; \text{kJ} \cdot \text{mol}^(-1)`

Therefore

`\Delta H = 678 - 862 = -184\; \text{kJ} \cdot \text{mol}^(-1) \; \text{Reaction}`